1. Field of the Invention
The present invention relates to a liquid circulation unit which is used for a liquid jet coating apparatus to jet and coat a droplet to an object to be coated, a liquid circulation apparatus and a method of manufacturing a coated body.
2. Description of the Background
A droplet jet coating apparatus is used for printing an image information, and in addition, for manufacturing various flat type display devices such as a liquid crystal display device, an organic EL (Electro Luminescence) display device, and an electron emission display device, a plasma display device, an electrophoresis display device and so on. The droplet jet coating apparatus is provided with a droplet jet head (an ink jet head, for example) to jet a liquid such as an ink and so on from a plurality of nozzles as droplets, makes the droplets land on an object to be coated such as a substrate and so on by the droplet jet head, forms a dot row of a prescribed pattern sequentially, and manufactures various coated bodies. The ink is supplied from an ink tank to the droplet jet head via a pipe arrangement (an ink flow path). The pipe arrangement is provided with a valve and a pump and so on. The liquid pressure of the ink inside the droplet jet head is kept to a negative pressure so as to prevent the leakage of the ink and so on from the nozzle (refer to Patent Document 1, for example). In such a droplet jet coating apparatus, as the ink including a material difficult to dissolve is used, the sedimentation of the material occurs by the deterioration with age of the ink, and as a result, a jet failure caused by the sedimentation is generated. To solve the problem, a droplet jet coating apparatus is proposed to circulate an ink between a droplet jet head and an ink tank (refer to Patent Document 2, for example).
However, in the above-described droplet jet coating apparatus, as a pressure fluctuation is applied to the droplet jet head via the ink inside the pipe arrangement by driving the valve and the pump, the leakage of the ink and the suction of the air and so on occur. For the reason, the leakage of the ink and the suction of the air bubbles by the droplet jet head occur, and as a result, a jet failure such as non-jet and so on may occur. Thus, a droplet jet coating apparatus capable of suppressing the occurrence of the jet failure is also proposed (refer to Patent Document 3, for example).
FIG. 7 is a diagram for explaining each unit relating to the ink circulation in the droplet jet coating apparatus stated in Patent Document 3. As shown in FIG. 7, a droplet jet head H is provided with an internal flow path Ha through which an ink supplied from a liquid storage unit 121 passes, and jets the ink passing through the internal flow path Ha from a nozzle N as droplets. A first buffer tank 119 formed so that the ink flowing from a flow path 131a of a liquid supply flow path 131 drops in the first buffer tank 119 is provided in the liquid supply flow path 131 positioned closer to the liquid jet head H side than a liquid supply unit P1. In addition, a second buffer tank 120 formed so that the ink flowing from a flow path 132a of first and second liquid return flow paths 132, 133 drops in the second buffer tank 120 is provided in the first and second liquid return flow paths 132 and 133 positioned closer to the liquid jet head H side than liquid return units P3, P4. By this, the pressure fluctuations caused by driving the liquid supply unit P1 and the liquid return units P3, P4 are absorbed by each of the air layers of the first buffer tank 119 and the second buffer tank 120, so that the occurrence of the leakage of the ink and the suction of the air and so on caused by the pressure fluctuations can be suppressed. In addition, as the ink circulates through the liquid supply flow path 131, the internal flow path Ha of the droplet jet head H, the first liquid return flow path 132 and the second liquid return flow path 133, the sedimentation of the material included in the ink can be suppressed. For these reasons, the jet failure caused by the sedimentation of the materials in the ink can be suppressed, and in addition, the jet failure caused by the exudation of the liquid or the suction of the air bubbles can be suppressed.
Next, a circulation route in a conventional and general droplet jet coating apparatus will be described. As shown in FIG. 8, a conventional droplet jet coating apparatus is provided with a supply tank 1, a liquid sending pump 2, a deaeration unit 3, a filter 4, a sub-tank 5, a head 6, a return pump 7, a discharge tank 8, and a chemical solution valve 9. The sub-tank 5 shown in FIG. 8 corresponds to the first buffer tank 119 shown in FIG. 7, and the discharge tank 8 shown in FIG. 8 corresponds to the second buffer tank 120 shown in FIG. 7. Diaphragm pumps are used for the pumps in the route, and diaphragm valves are used for the valves in the route. The circulation of the ink is achieved by repeating four actions of (A1) liquid sending, (A2) discharge, (A3) return and (A4) bypass circulation. (A1) liquid sending is an action to absorb the ink stored in the supply tank 1 by the liquid sending pump 2, to make the ink pass through the filter 4 and the deaeration unit 3, and to supply the ink to the sub-tank 5. (A2) discharge is an action to pressurize the sub-tank 5 so as to send the ink with applied pressure through an IN side to an OUT side of the head 6, and thereby to discharge the ink to the discharge tank 8. As the route inside the head 6 is the same as shown in FIG. 7, the detailed description is omitted here. The discharge action can be achieved, besides the action to pressurize the sub-tank 5, by an action to absorb the ink by making the discharge tank 8 in the negative pressure by the return pump 7. (A3) return is an action to absorb the ink discharged to the discharge tank 8 by the return pump 7 and to return the ink to the supply tank 1. (A4) bypass circulation is an action to circulate the ink in the bypass route while removing the air bubbles by the deaeration unit 3, as the material included in the ink may settle out in the route when the head is detached. The deaeration unit 3 is, specifically, a pipe arrangement for the deaeration to remove the dissolved gas in the ink using hollow fiber films. The circulation flow rate is not more than 10 ml/min so as to keep the deaeration ability.
Patent Document 1: Japanese Patent Disclosure (Kokai) P2006-192638
Patent Document 2: Japanese Patent Disclosure (Kokai) P2004-230652
Patent Document 3: Japanese Patent Disclosure (Kokai) P2008-264767
However, in the droplet jet coating apparatus stated in the above-described Patent Document 3, there is a problem described below.
Firstly, there is a problem that the pipe arrangement route is complicated. For the reason, in case of the ink exchanging operation, it was necessary to stop the apparatus for a long time so as to clean the pipe arrangement and to exchange the ink. In addition, the ink exchanging operation itself was very high in operation difficulty level, as it was necessary for an operator to enter into the apparatus. In addition, as it is necessary to withdraw the ink existing in the pipe arrangement route as waste liquid, so that the losses of the ink and the cleaning liquid were high.
In addition, there is a problem that in the discharge action, while the ink in the head is sent with applied pressure by pressurizing the first buffer tank, the ink may be discharged from the nozzle side, too. (By applying the pressure of 20 kPa in 6 seconds, the ink is discharged by about 3 cc.) As the discharge actions are performed at a frequency of once two hours, the loss amount per day becomes very large such as 3×12=36 cc, and thus much running cost is required.
In addition, there is a problem that in the circulation action when the first buffer tank is pressurized, as the head can not coat during the circulation action, the head is to be evacuated to the evacuation position. In this case, the circulation action (10 minutes are required for one cycle of liquid sending→discharge→return→bypass circulation) is required for 10 minutes per 2 hours to suppress the sedimentation of the ink, and thus the operating time of (10×12=) 120 (=10×12) minutes per a day is lost. In addition, as the line balance between the former process and the later process can not be held, (the substrate inputted from the former process can not be treated and thus becomes a backlog) the time operation availability of the line may deteriorate.